The present invention relates to frequency source devices, particularly useful for effecting radio frequency broadcasting through the fast sweep over a series of pre-determined frequencies.
In various applications, especially in certain communications systems, there is need for stable frequency sources which allow for very fast switching between frequencies.
For example, various radio frequency broadcast systems are available which are designed to make it difficult or impossible to electronically jam the broadcast. One way of accomplishing this is to broadcast sequentially over a number, preferably a large number, of frequencies, switching rapidly from one frequency to the next. In such a system, signals are typically broadcasted at one frequency for only a short period of time, before switching to a different frequency.
If the switching speed is sufficiently great, comprehensible transmitted information, such as human speech, may be reconstructed using partial groups of a relatively large number of reception frequencies. In this way, if one or more frequencies are being jammed, the information can still be received using the frequencies which are not being jammed without loss of information.
Two basic techniques for generating ultra fast frequency switching are in current use. One method involves the use of a Direct Digital Synthesizer (DDS), while another involves the use of two parallel frequency generators operating in tandem.
A DDS synthesizes frequencies directly by repetitive arithmetic manipulations of frequency in a series of modules. Frequency synthesizers combine the advantages of tunable oscillators, which are not usually highly stable, with those of frequency standards, which, though very stable, are not tunable.
The typical DDS suffers from at least two serious disadvantages. First, the maximum output frequency of a typical DDS is approximately 10 to 15 MHz. In order to achieve higher frequencies a highly complex radio frequency (RF) processing system is required. Second, the noise level of the output frequency of a DDS is relatively high.
Use of two frequency generators in tandem, such that when one generator is producing signals the other is stabilizing so as to be ready to produce the next frequency upon switching, yields very good frequency generation performance. However, this performance is achieved at a very high cost, both in terms of the required hardware and in terms of the physical volume occupied by the dual system. In certain applications, either the cost or the volume, or both, may be critical factors.
There is thus a widely recognized need for, and it would be highly advantageous to have, a small and inexpensive frequency source which will reliably and very quickly sweep over a series of frequencies.